Plasticizers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, toys, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticizers include polyvinyl butyral, acrylic polymers, poly(vinylidene chloride), nylon, polyolefins, polyurethanes, and certain fluoroplastics. Plasticizers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers). A listing of the major plasticizers and their compatibilities with different polymer systems is provided in “Plasticizers,” A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp. 157-175.
Plasticizers can be characterized on the basis of their chemical structure. The most important chemical class of plasticizers is phthalic acid esters, which accounted for about 85% worldwide of PVC plasticizer usage in 2002. However, in the recent past there as been an effort to decrease the use of phthalate esters as plasticizers in PVC, particularly in end uses where the product contacts food, such as bottle cap liners and sealants, medical and food films, or for medical examination gloves, blood bags, and IV delivery systems, flexible tubing, or for toys, and the like. For these and most other uses of plasticized polymer systems, however, a successful, general purpose substitute for phthalate esters has heretofore not materialized on a commercial scale.
One such suggested substitute for phthalates are esters based on cyclohexanoic acid. In the late 1990's and early 2000's, various compositions based on cyclohexanoate, cyciohexanedioates, and cyclohexanepolyoate esters were said to be useful for a range of goods from semi-rigid to highly flexible materials. See, for instance, WO 99/32427, WO 2004/046078, WO 2003/029339, WO 2004/046078, U.S. Application No. 2006-0247461, and U.S. Pat. No. 7,297,738.
Other suggested substitutes include esters based on benzoic acid (see, for instance, U.S. Pat. No. 6,740,254, and also co-pending, commonly-assigned, U.S. Provisional Application 61/040,480, polyketones, such as described in U.S. Pat. No. 6,777,514; and also co-pending, commonly-assigned, U.S. Provisional application Ser. No. 12/058,397, and triglycerides, such as described in co-pending, commonly assigned, U.S. Provisional Application 61/040,490. Epoxidized soybean oil (ESO), which has much longer alkyl groups (C16 to C18) has been tried as a plasticizer, but is generally used as a PVC stabilizer in low concentrations. At higher concentrations, ESO exudaton can occur.
Typically, the best that has been achieved with substitution of the phthalate ester with an alternative material is a flexible PVC article having either reduced performance or poorer processability. Thus, heretofore efforts to make phthalate-free plasticizer systems for PVC have not proven to be entirely satisfactory, and this is still an area of intense research.
U.S. Pat. No. 2,233,513 teaches aroylbenzoic acid esters with nitrocellulose and acetylcellulose. Nitrocellulose and acetylcellulose are resins used in centuries-old technology and find only limited use today. These materials are very brittle without plasticizer. The most common plasticizer for these resins was camphor. In part because of the odor imparted to the final product caused by the use of camphor, there were constant efforts to find alternative plasticizers. In general for every polymer, you need to have a plasticizer with the correct balance of solvating properties, volatility, and so forth. In the case of nitrocellulose, most of these efforts to find replacement plasticizers were in the area of improving the processability, stability, and decrease the brittleness of rigid or semi-rigid nitrocellulose products. The first applications of nitrocellulose were for ivory substitutes in billiard balls, false teeth, and piano keys. Here the plasticizers help greatly in processing and to reduce the brittleness of these rigid products. Later applications of nitrocellulose were in the area of stiff brush or combs, which had, before the use of nitrocellulose, been made from natural products. Eventually nitrocellulose found use in motion picture film. However, while “plasticizing” such resins made them more impact resistant and durable, this technology was rapidly replaced, over the span of barely a decade, with the introduction of PVC-based resins. In contrast to the cellulosic resins, PVC may be made truly flexible by plasticizing with the appropriate materials. Accordingly, there is no reason to assume that a plasticizer used with cellulosic material can be used successfully with PVC. Camphor, for instance, is not a good plasticizer of PVC. The same is seen with plasticizers used in polyvinyl butryal (PVB)—generally successful plasticisers of PVB resin are not useful in PVC. Plasticizers used in these polymers are not necessarily good plasticizers.
Accordingly, the industry still seeks a general purpose non-phthalate plasticizer, particularly a plasticizer that has a suitable melting or pour point, increased compatibility with the resin, and providing a PVC composition having good performance and low temperature properties, wherein the plasticizer can be made by a process having a high throughput and using readily available raw materials.
In U.S. Pat. No. 2,372,947, alkyl esters of ortho-henzoyl benzoic acid are described as being useful in polyvinyl halide resins.
The present inventors have surprisingly discovered that keto acid esters have advantageous properties when used in PVC and furthermore can readily made by esterifying alcohols with keto acids, the keto acids preferably being derived by acylating an aromatic molecule with a cyclic anhydride.